Francium is so reactive, it would react with the moisture in the air. Aside from that, it is also very hard to find. Experienced researchers believe that there are hardly 30 grams of francium found on the surface of the earth.

Francium is highly radioactive so it throws off a lot of heat, so much so infact that IF we could even find a way to get enough francium in one place to form a piece large enough to see, it would immediately vaporize from its own heat.

They wouldn't react.
Francium has actually never been observed. It just falls apart too fast (the most stable isotope has a half-life of 22 minutes). Any usable quantity of Francium vaporizes almost instantly through beta decay which would take precedence over reaction with Fluorine.

Not as big a bang as you'd think, but it does depend on the concentrations. If you were to get a disk of Caesium (which is harder to do than you think) so that a lot of it's surface is exposed to Fluorine at once, you'd get a very impressive explosion. Otherwise, it would be a very violent but not explosive reaction. It would cause the vessel to jump around a lot and probably crack.

If you could get enough Francium in one place (and as wheresthefudge pointed out, that's just theoretical,) presumably you would get an exponentially bigger bang.

Nope. Cesium would explode due to contact with the water vapor in the air. That is sodium or lithium. Maybe potassium. Francium and cesium have to be contained in oil otherwise they react with simple air.

Cesium only exists in a few hundredths of a gram at any one point on Earth. Most of it is synthetically made in labs. The reaction as you move down the Alkali metals increases by a factor of ten for each one. Since cesium is at the bottom, it's reactive potential is far greater than what is shown. The table would have been shattered.

And the people who do have it are unwilling to use what little they must have to demonstrate and essentially useless demonstration. If I had some of the only available Francium, I certainly wouldn't waste it on this. Someone should make a simulation of what it should look like.

It's not necessarily that Francium is rare, it's more that it doesn't exist for any appreciable timeline (falls apart from beta decay)
Fun fact; nobody knows what color francium is because they've never gotten enough of it together to look at.

guies!.....guies..... I know you're sciencing and all about elemental reactivity and junk, but.....guies....No, guies seriously.....Explosion.... Mildly entertaining explosion. Can we just enjoy that for a moment? And maybe some thumbs for their troubles?